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Polarizer
A polarizer is a filter that only allows light with a specific orientation to pass through it. There are two types of polarizing filters - linear and circular - and there are a lot of misconceptions and confusion in understanding the differences between the two, and which one is right for you. Basics Before you can understand the difference between types of polarization, you must first understand what polarization is, and that requires a basic understanding of the properties of light. A detailed understanding of light and polarization would require knowledge of electromagnetic theory - which I won't get into - but a basic understanding can be had without getting carried away. Light is an electromagnetic wave. It travels as a wave form. Most of the primary light sources you'll encounter in photography - sun, light bulbs, strobes - are unpolarized. That is, the EM waves that make up the light travel equally and randomly in all directions away from the source. Linear Polarization The term "polarization" refers to the orientation of the electric field of an electromagnetic wave. When the electric field vector is oscillating in a single plane, it is defined as linear polarization. Linear polarization commonly occurs in nature when light is reflected off of a surface, such as a wall or a lake. Reflected light is often polarized at the angle of the surface it is reflected off of. If it is the surface of the lake, the light will be polarized horizontally. If a vertical wall is the reflector, the light will be polarized vertically. This is the technology behind glare-reduction filters or "Blue-Block Sunglasses." The polarized lenses can block out this reflected light since it tends to be polarized the same way. A linear polarizing filter allows only light that is oriented a certain way to pass through. Because any photon can be oriented to any angle, most photons will probably not be oriented in the direction that the lenses allow. Thus, linear filters can block as much as 60% of the light from reaching the film. Circular Polarization The concept of circular polarization is similar to that to linear polarization. Circular polarization is a combination of two perpendicular linear waves that are 90 degrees out of phase with each other. As any light wave can be represented as a sum of two linearly polarized waves, it also can be sought of as a sum of two circularly polarized waves, one rotating clockwise and another – counterclockwise. In physics, the circular polarizing filter is a device which when illuminated by wave containing both clockwise and counterclockwise components, would pass only one of them. The ‘circular polarizer’ term when applied to filters used with modern photo cameras is misleading, because it does not do what is described above, namely, allowing to pass only one circular polarization. Which is good, because it is not what is required in this case. The circularly polarizing filter in photography consists of two components: first light passes though a regular linearly polarizing filter which allows only one linear polarization to pass and blocks another, and then light goes through a second component – a quarter wave plate which converts the linearly polarized light into a circularly polarized. The first component allows to block or highly reduce light scattered from a sky at 90 deg. and reflections from shiny surfaces such as glass or water. The second component is needed for most of the modern cameras to work with the resulting light correctly. If your camera uses beam-splitter reflecting all or part of light at 90 deg. into auto-focusing device and into an exposure-metering system (e.g. if you have am SLR or DSLR camera), then it utilizes only one linear polarization out of incoming light. Both systems are designed to work with light which is not linearly polarized. However, if we place a linear polarizer in front of a camera, this light might be totally blocked by 90-deg reflection, and both exposure meter and auto focus will not work properly. So, to make the camera work after passing the light through a linear polarizer, we need to convert this light into a state which contains both linear polarizations in equal amounts. This is done with the use of a quarter-wave plate, which produces a circular polarized beam, which allows your SLR camera to function properly. Choosing The Right Filter Linear polarizing filters seem to have a greater effect on the final image than circular polarizers do, and in spite of many misconceptions, you can use either type of filter on any camera. But there is a catch: if you use a linear polarizer on a modern camera, you may not be able to use your built-in metering, or autofocus, or possibly both. Modern autofocus systems use mirrors that act as beam-splitters: most of the light is reflected to the viewfinder for metering and viewing, while the rest (typically 25%) is transmitted and then reflected by a secondary mirror to the autofocus sensor, which is in the camera body. With a polarizing filter attached, the ratio of reflected/transmitted light (fixed for non-polarized light) varies with the polarization plane orientation of polarized light, relative to the beam-splitter, causing the amount of light reaching the autofocus sensor to vary. This effect can be particularly troublesome if your front element moves with the lens, as it would cause the amount of light reaching the sensor to change while the lens focuses. While you can see (rather easily) when your autofocus system has been confused by a linear polarizer, there isn't such an easy way of knowing whether or not the internal light metering system has been stumped. The same loss of light principles that affect the autofocus system can also affect a camera's built-in meter, and obviously this is never good. What use is a light meter if it receives the wrong amount of light? In both situations, a circular polarizer corrects the problem. Having removed the effect of linear polarization, the light functions normally inside the camera, in the beam splitter, and reaching the autofocus and metering sensors properly. Therefore, for your convenience, you should use a circular polarizer if your camera has autofocus or built-in light metering features. However, if you focus manually, and use a hand-held light meter, you can use a linear polarizer on any camera you wish. Uses For Polarizing Filters Polarizing filters have a number of uses, among them: * Reducing haze. Since light comes from all over, at random polarizations, adding a polarizing filter will block the crossing polarization patterns of scattered and reflected light to reduce haze. * Removing reflections. As mentioned above, light is naturally polarized when it is reflected, so by rotating your polarizing filter to cross-polarize the reflected light, you can effectively remove the reflection from the photo. * Increasing color saturation. By reducing haze from scattered, random light, saturation of color is dramatically improved. The effect is particularly evident in skies and in foliage. * Neutral density. The natural loss of 1-2 stops of light, without shifting color, makes a polarizing filter a good ND filter as well. Notes About Polarizing Filters Here are a few other things to keep in mind when using polarizing filters: * When stacking filters, the polarizer should always be the last (outermost) filter on the stack. Passing through a polarizing filter further down the chain may alter the results of other filters you had used. * Lenses with rotating front elements will mess up your polarizing filter's rotation. If your lens has a rotating front element, you should adjust the polarizing filter after you've set the focus. * The effect of polarization (outdoors) is most prominent at a 90-degree angle to the sun. * The angle of polarization varies continuously with the angle from the sun. With a lens wider than about 28mm (in 35mm film terms) the sky will be unevenly polarized. If you like the effect, go ahead and use it. Links * DP FWIW page on using a polarizer effectively * A University of Pennsylvania lecture on propagation of light * Information about polarization found at the Southern California Earthquake Center * Luminous Landscape article: Understanding Polarizers Category:Lens accessories